Two main features of vocabulary knowledge, namely breadth and depth, have a fundamental role in vocabulary research. This research aimed to study the relationship between vocabulary knowledge and reading comprehension, and to investigate which feature of vocabulary knowledge, breadth or depth, had better impact on identifying reading comprehension performance. Therefore, three language tests were used, including a reading comprehension test, Vocabulary Levels Test, and Word Associates Test. 64 students majoring in Medicine at Ferdowsi University of Mashhad, Iran, participated in the study. The outcomes of the two-tailed Pearson Correlations and multiple regression analyses declared that test results on vocabulary breadth, depth of vocabulary knowledge, and reading comprehension were positively correlated and breadth and depth of vocabulary knowledge were closely interrelated.